One known millimeter waveguide using anisotropically etched silicon substrate is disclosed in IEEE MTT-S Digest pp. 797-800, 1996.
FIG. 10 shows the structure of a conventional millimeter wave transmission line. Silicon dioxide (SiO.sub.2 ) 902 is deposited on a silicon substrate 901, and a microstrip line 903 is formed on the silicon dioxide 902. A shielded microstrip line is created by sandwiching the silicon substrate 901 between a carrier substrate 904 coated with metal film, and another silicon substrate 905 processed by micromachining, to achieve a shielding structure. With this shielding structure, which uses air as the dielectric medium, a transmission line with low loss can be achieved.
In this type of millimeter transmission line , however, modularization by mounting other millimeter wave components such as an MMIC (Monolithic Microwave Integrated Circuit) may be difficult, because the microstrip line is supported by silicon dioxide in midair. There may also be a problem with strength. Two sheets of silicon substrate are processed by micromachining, and an unduly thick silicon dioxide film must be formed to ensure strength. These result in the need for complicated processing during manufacturing.